As typical examples of the prior heat resistant amorphous resin, aromatic polyether-sulfone (for example, Victrex PES.RTM., manufactured by ICI), polyether-imide (for example, Ultem.RTM., manufactured by GE) and the like are known. Although these resins are excellent heat-resistant engineering resins having a high glass transition point and usable at high temperatures, since they have highly polar groups such as sulfone group or imide group in their molecule, their performance is greatly affected by moisture absorption. In addition, these resins have a high dielectric constant. Thus, their use in the electrical industry is inevitably restricted. Further, since the sulfone group and imide group are poor in heat stability, these polymers are insufficient in stability when molded at high temperatures, and can undergo foaming, decomposition, coloration, etc., during the molding operation.
On the other hand, as resins having better heat stability, various aromatic polyether-ketones comprising only aromatic rings, ketone groups and ether groups are known. For example, there are known Victrex PEER.RTM. manufactured by ICI, polymers represented by general formula (I): ##STR2## wherein the value of x varies, polymers prepared by introducing biphenylene structure or naphthalene structure thereinto, and the like. However, all of these resins are made of a crystalline polymer, and any amorphous totally aromatic polyether-ketone is not yet known. Although these crystalline resins are excellent in heat resistance as mentioned above, they are insufficient in dimensional stability and inferior in high-temperature stiffness due to their low glass transition temperature.
Aside from the above, polymers having a dibenzofuran skeleton prepared by allowing dibenzofuran to react with tere(iso)phthalic acid chloride in the following manner have also been reported. However, the polymers obtained by this method are all low in molecular weight, or fragile and colored. ##STR3##
That is, there is disclosed a trimer in J. Appl. Polym. Sci. 9, (5) 1955 (165). There is disclosed a low molecular weight polymer of which reduced viscosity was 0.18 dl/g in U.S. Pat. No. 3,441,538. And there is disclosed a deep red-colored brittle film in Example 3 (Comparative Example) of Japanese Patent Unexamined Publication Kokai (Laid-Open) No. 63-399231.
In Japanese Patent Unexamined Publication Kokai (Laid-Open) No. 63-399231, this was deduced to be attributed to the fact that by the Friedel-Crafts method, an acrylation was not restricted the para position but it took place also in the ortho position, which activated the oxygen group, thereby caused branching and coloration.
As mentioned above, any dibenzofuran skeleton-containing polymer having a high molecular weight and a high stiffness has not been obtained.
In Japanese Patent Unexamined Publication Kokai (laid-Open) No. 61-211336, a production of a polymer by Friedel-Crafts reaction was described, where dibenzofuran was referred to as one of the 15 monomers used as nucleophilic copolymerizable components, and oxybis(4,4'-benzoyl chloride) was referred to as one of the 27 monomers used as electrophilic copolymerizable components. However, there is nothing disclosed in the specification as to a reaction of the two monomers in combination and the polymer formed by the reaction.